Hornsby Shire Council
Attachment to Report No. WK34/09 Page 0
SHEPHERDS DRIVE, CHERRYBROOK
ROUNDABOUT AT KENBURN AVENUE
The redevelopment of Cherrybrook Village Shopping Centre in 2005 resulted in increased traffic delays in Shepherds Drive. Traffic attempting to enter Cherrybrook Shopping Village is delayed by vehicles slowing to access parking spaces within the centre. To manage the problem a left turn slip lane into the shopping centre was provided in August 2008. A review by Traffic and Road Safety Branch completed in March 2009 has confirmed that delays have been significantly reduced and the roundabout is operating in accordance with its design parameters. Additional improvements cannot be justified using only traffic management criteria.
Despite the recent improvements to the roundabout some motorists consider the design too restrictive. Council continues to receive correspondence concerning the operation of the roundabout. Further changes to widen the roadway will create an environment where vehicles can negotiate the roundabout at speeds exceeding the current speed limit in Shepherds Drive, nullifying the principal road safety benefit of the roundabout.
This report has been prepared to review the design and operation of the roundabout following modifications to reduce traffic delays as per the resolution of reports WK49/07 and WK22/08, and to review options regarding the likely cost and benefit of any further work.
Current roundabout arrangement.
A number of site inspections in 2006 and 2007 confirmed that the single lane roundabout was prone to congestion caused by vehicles attempting to enter the carpark queuing back into Shepherds Drive in both directions, causing delays on all legs of the intersection. However, while queuing did occur during traffic peaks, the extent and duration of the queuing fell below the level required to justify major reconstruction. Traffic management practice and Roads and Traffic Authority (RTA) modelling standards require analysis of a continuous hour of traffic data to quantify and compare delays across the Sydney area. Therefore locations experiencing delays of less than one hour continuous duration are not suitable for cost benefit analysis and will not qualify for state or federal funding assistance. Typically queues in Shepherds Drive would form then disperse a number of times within a one hour period, with the worst periods around mid day and 5 pm weekdays and Saturday mornings.
Consultation between shopping centre management and Council resulted in a number of changes to the shopping centre car park commencing late 2005 which reduced but did not eliminate queuing into the roundabout. In addition shopping centre management agreed to contribute 50% towards the design and construction of “low cost” improvements to the roundabout which it had no obligation to do as Council has accepted the traffic assessment provided with their development application for the refurbishment completed in 2005. At that time the developer’s traffic consultant provided a report which showed that anticipated delays post redevelopment would be within the acceptable range for traffic efficiency for the Sydney region, therefore Council was unable to impose additional consent conditions on the developer in relation to access. The veracity of the consultant’s data has since been confirmed by Council after the redeveloped site commenced operating. Council’s traffic data confirmed that traffic management standards were being met despite continuing complaints from the public regarding the roundabout. Nevertheless, Council considered report WK49/07 and resolved to investigate options.
In 2008 Council prepared a proposal to provide an extra left turn slip lane to “desirable” traffic engineering standards which required adjustment of a Telstra pit and property acquisition. The cost of this work was estimated to be $270,000 excluding property acquisition. This proposal was “Option 2” as reported to Council in WK22/08. As an alternative, a proposal avoiding adjustment of the pit or property acquisition was prepared using “minimum” traffic engineering standards. The cost of this proposal, “Option 1” in WK22/08, was $45,000 and this option was adopted by Council and referred to the Local Traffic Committee (LTC) for a recommendation.
It should be noted that the LTC operates under RTA guidelines and has no decision making powers. Council decides which proposals it will refer to the LTC, based on Council’s own funding priorities, while the LTC’s role is to recommend whether these proposals meet acceptable design, operation and enforcement standards. The LTC report did advise that an alternative proposal meeting “desirable” standards had been considered by Council but was not adopted due to excessive cost.
Accordingly the LTC recommended that the low cost left turn lane proposal using minimum design standards was acceptable and should be provided. In determining whether the use of minimum standards is appropriate in this situation, it must be recognised that the additional left turn lane was provided to store vehicles waiting to enter the shopping centre carpark without obstructing through traffic during traffic peaks. While the lane widths are to minimal standard they are sufficient to allow through vehicles to slowly pass stopped vehicles. The approved design was developed using Australian Standard vehicle templates and allows two cars to enter the roundabout at slow speed. Where the left turning vehicle does not keep left, or either vehicle is larger than the design vehicle, the other vehicle has the option of using the mountable portion of the roundabout which is designed specifically to allow vehicles to cross at slow speed. This creates a low speed environment which is appropriate for the level of traffic congestion and pedestrian conflict occurring during peak periods. Whether or not the queue forms is dependant on motorist’s behaviour within the carpark and no amount of work on Shepherds Drive or the roundabout can influence the need to queue within the carpark.
Traffic data obtained between
Attachment 4 shows the relative queue lengths before and after the additional lane was added. A queue is defined as occurring when vehicles travel at 10 km/h or less and for the most part these queues consist of moving vehicles. It should be noted that even the worst queues measured in 2007 and reported in LTC12/2008 equated to a Level of Service C. Level of Service is a traffic management performance standard which uses quantitative data to provide a qualitative assessment of roads and intersections. There are 6 Levels of Service ranging from Level of Service A (minimum delay) to Level of Service F (worst delay with traffic arriving faster than it can leave). Level of Service C is considered satisfactory for new work in the Sydney Region therefore the current arrangement, at Level of Service B, is better than merely acceptable.
Council’s Design and Construction Branch have investigated 2 options relating to the operation of the roundabout however it needs to be recognized that the queues in Shepherds Drive forming to enter the shopping centre carpark are initiated within the carpark and no changes to the roundabout in Shepherds Drive will reduce queues.
Option 1 - This option is the alternative considered but not adopted in 2008 due to the higher cost. It provides wider traffic lanes by relocating a Telstra pit and acquiring land for an estimated cost exceeding $300,000. Queuing and delays would not change. The benefit would be a reduction in complaints from the public wanting faster movement through the roundabout. The disbenefit would be that vehicles, in particular through vehicles, could enter the roundabout faster than currently, leading to more complaints of vehicles not giving way, more complaints from pedestrians about difficulty in crossing Shepherds Drive, more incidents of high range speeding and possibly more crashes and crashes of greater severity. Refer to Attachment 2 for a copy of this option.
Option 2 – A less costly option to improve manoeuvring through the roundabout involves narrowing the central roundabout island by about 1 metre to create a wider through lane within the roundabout, without changing the kerb alignment outside the child care centre. The estimated cost would be $50,000. Queuing and delays would not change and the benefits and disbenefits are the same as above. Refer to Attachment 3 for a copy of this option.
It should be noted that both options severely compromise the speed
reduction benefits of the roundabout outside peak times. The speed limit in
The Traffic Officer Eastwood Police advises that the Police will not support any changes to the roundabout which will result in increased speeds. Other sections of Shepherds Drive have a history of after hours speeding complaints and the roundabout in its current form has proven to be an effective speed management device with only one collision reported in the last 12 months.
Cherrybrook Village Shopping Centre management has advised it is unable to consider financially contributing to further reconstruction at this stage as it considers the left turn slip lane has reduced delays and complaints significantly. Centre management will continue to consider changes to the internal layout of the carpark to improve circulation and further reduce queuing into Shepherds Drive.
The following options have not been considered in any detail and will require extensive design and consultation if Council requires accurate estimates.
Option 3 – Construct a new roundabout with 2 circulating lanes. The geometry of the intersection is currently appropriate for a single lane roundabout only. To provide a second lane will require almost total reconstruction due to pavement cross falls, necessitating extensive property acquisition from all corner properties (and perhaps further), and additional public utility relocation. Pedestrian access will be severely affected by increased speeds. Based on the estimate for Option 1, the cost of this work will exceed $500,000.
Option 4 - Council may consider signalising the intersection, either by signalising the roundabout or removing the roundabout and reconstructing the intersection. The cost of either would exceed $200,000. Neither option can be recommended as queues entering the carpark will continue to form back into the intersection, preventing vehicles from entering the intersection. In addition, traffic signals will result in vehicle delays occurring outside peak periods as well as during peak periods. Signals may, however, improve pedestrian access during peak periods. Assuming Council could obtain RTA approval for the traffic signals, Council would have to fully fund traffic signals as they do not meet the RTA warrants for traffic volumes, pedestrian volumes or crashes. As stated earlier, the volumes and delays have to be measured over an hour, and the required volumes have to be exceeded for four hours in total each day.
The provision of the slip lane in mid 2008 resulted in the roundabout operating in accordance with the design parameters which included reducing delays during peak periods without increasing after hours speeding, at a construction cost acceptable to Council. Delays currently experienced on Shepherds Drive are being monitored and are within the range traffic management guidelines consider acceptable throughout Sydney Region during peak periods.
Further changes to the roundabout cannot be recommended given that the current layout meets the design goals and further changes may have adverse road safety implications, particularly for pedestrians. Despite the level of complaints received recently regarding the operation of the roundabout, on site observations and available traffic data indicates that the intersection of Shepherds Drive and Kenburn Avenue operates in accordance with prescribed traffic management performance standards.